U.S. patent number 9,653,731 [Application Number 14/580,305] was granted by the patent office on 2017-05-16 for layered oxide materials for batteries.
This patent grant is currently assigned to Sharp Kabushiki Kaisha. The grantee listed for this patent is Sharp Kabushiki Kaisha. Invention is credited to Robert Gruar, Emma Kendrick.
United States Patent |
9,653,731 |
Kendrick , et al. |
May 16, 2017 |
Layered oxide materials for batteries
Abstract
Materials are presented of the formula:
A.sub.xM.sub.yM.sup.i.sub.ziO.sub.2-d, where A is sodium or a mixed
alkali metal including sodium as a major constituent; x>0.5; M
is a transition metal; y>0; M.sup.i, for i=1, 2, 3 . . . n, is a
metal or germanium; z.sub.1>0 z.sub.i.gtoreq.0 for each i=2, 3 .
. . n; 0<d.ltoreq.0.5; the values of x, y, z.sub.i and d are
such as to maintain charge neutrality; and the values of y, z.sub.i
and d are such that y+.SIGMA.z.sub.i>1/2(2-d). The formula
includes compounds that are oxygen deficient. Further the oxidation
states may or may not be integers i.e. they may be whole numbers or
fractions or a combination of whole numbers and fractions and may
be averaged over different crystallographic sites in the material.
Such materials are useful, for example, as electrode materials in
rechargeable battery applications.
Inventors: |
Kendrick; Emma (North
Warnborough, GB), Gruar; Robert (Swindon,
GB) |
Applicant: |
Name |
City |
State |
Country |
Type |
Sharp Kabushiki Kaisha |
Osaka |
N/A |
JP |
|
|
Assignee: |
Sharp Kabushiki Kaisha (Osaka,
JP)
|
Family
ID: |
56130493 |
Appl.
No.: |
14/580,305 |
Filed: |
December 23, 2014 |
Prior Publication Data
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Document
Identifier |
Publication Date |
|
US 20160181607 A1 |
Jun 23, 2016 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M
4/525 (20130101); C01G 51/50 (20130101); C01G
49/0072 (20130101); H01M 4/505 (20130101); C01G
53/50 (20130101); Y02E 60/10 (20130101); C01P
2006/40 (20130101); C01P 2002/72 (20130101); H01M
10/054 (20130101); C01P 2006/32 (20130101); C01P
2002/88 (20130101) |
Current International
Class: |
H01M
4/525 (20100101); C01G 53/00 (20060101); C01G
51/00 (20060101); H01M 4/505 (20100101); C01G
49/00 (20060101); H01M 10/054 (20100101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2506859 |
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Apr 2014 |
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GB |
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2006-179473 |
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Jul 2006 |
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JP |
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2010 129509 |
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Jun 2010 |
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JP |
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5085032 |
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Nov 2012 |
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JP |
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2012-252962 |
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Dec 2012 |
|
JP |
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2015 118898 |
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Jun 2015 |
|
JP |
|
WO 2006/057307 |
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Jun 2006 |
|
WO |
|
WO 2009/099061 |
|
Aug 2009 |
|
WO |
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WO 2013/140174 |
|
Sep 2013 |
|
WO |
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WO 2015/177568 |
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Nov 2015 |
|
WO |
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Other References
Ono et al., "Thermoelectric properties of Ca-doped
.gamma.-Na.sub.xCoO.sub.2," Transactions of the Materials Research
Society of Japan, Sep. 2004, vol. 29, No. 6, p. 2821-2824 (2004).
cited by applicant .
H. Schettler, "Investigation of solid sodium reference electrodes
for solid-state electrochemical gas sensors", Applied Physics A,
1993, vol. 57, Issue 1, pp. 31-35. cited by applicant .
Komaba, S. et al., Electrochemical Na insertion and solid
electrolyte interphase for hard-carbon electrodes and application
to Na-ion batteries, Adv. Funct. Mater. 21, 3859-3867(2011). cited
by applicant.
|
Primary Examiner: Kopec; Mark
Assistant Examiner: Hammer; Katie L
Attorney, Agent or Firm: Renner, Otto, Boisselle &
Sklar, LLP
Claims
The invention claimed is:
1. A composition having the formula
A.sub.xM.sub.yM.sup.i.sub.ziO.sub.2-d, where A is sodium or a mixed
alkali metal including sodium as a major constituent;
0.5.ltoreq.x.ltoreq.0.76; M is nickel; y>0; M.sup.i.sub.zi, for
i=1, 2, 3 . . . n is M.sup.1.sub.z1M.sup.2.sub.z2M.sup.3.sub.z3 . .
. M.sup.n.sub.zn, and wherein each of
M.sup.1.sub.z1M.sup.2.sub.z2M.sup.3.sub.z3 . . . M.sup.n.sub.zn is
a metal or germanium; z1>0 zi.gtoreq.0 for each i=2, 3 . . . n;
0<d.ltoreq.0.5; the values of x, y, zi for i=1, 2, 3 . . . n,
and d are such as to maintain charge neutrality; and the values of
y, zi for i=1, 2, 3 . . . n, and d are such that
y+.SIGMA.zi>1/2(2-d).
2. A composition as claimed in claim 1 in which M.sup.i, for i=1,
2, 3 . . . n, is a transition metal, an alkali metal or an alkaline
earth metal.
3. A composition as claimed in claim 1 wherein M.sup.i, for i=1, 2,
3 . . . n, is selected from the group consisting of: nickel, iron,
cobalt, manganese, titanium, aluminium, magnesium and
zirconium.
4. A composition as claimed in claim 1 in which
0.5.ltoreq.x.ltoreq.0.67.
5. A composition as claimed in claim 1 in which
0.67.ltoreq.x.ltoreq.0.76.
6. A composition having the formula
A.sub.xM.sub.yM.sup.1.sub.ziM.sup.i.sub.ziO.sub.2-d, where A is
sodium or a mixed alkali metal including sodium as a major
constituent; 0.76.ltoreq.x.ltoreq.1.3; M is a nickel; y>0;
M.sup.1 is an alkali metal or a mixed alkali metal; z1>0;
M.sup.i.sub.zi, for i=2, 3, 4 . . . n is
M.sup.2.sub.z2M.sup.3.sub.z3M.sup.4.sub.z4 . . . M.sup.n.sub.zn,
wherein M.sup.2.sub.z2 is a transition metal and each of
M.sup.2.sub.z2M.sup.3.sub.z3M.sup.4.sub.z3 . . . M.sup.n.sub.zn, is
a metal or germanium; z2>0; zi.gtoreq.0 for each i=3, 4 . . . n;
0<d.ltoreq.0.5; the values of x, y, z1, zi for i=2, 3, 4 . . .
n, and d are such as to maintain charge neutrality; the values of
y, z1, zi, for i=2, 3, 4 . . . n, and d are such that
y+.SIGMA.1+.SIGMA.zi>1/2/(2-d); and the composition adopts a
layered oxide structure in which the alkali metal atoms A are
coordinated by oxygen in a prismatic or octahedral environment, and
in which the alkali metal atoms M.sup.1 adopts a coordination
complementary to that adopted by the other M.sup.i transition metal
element(s).
7. A composition as claimed in claim 6 in which M.sup.i, for i=3, 4
. . . n, is a transition metal, an alkali metal or an alkaline
earth metal.
8. A composition as claimed in claim 6 wherein M.sup.1 is selected
from the group consisting of: sodium, lithium, and a mixture of
sodium and lithium.
9. A composition as claimed in claim 6 wherein M.sup.i, for i=2, 3
. . . n, is selected from the group consisting of: nickel, iron,
cobalt, manganese, titanium, aluminium, magnesium and
zirconium.
10. A composition as claimed in claim 6 in which 0.9.ltoreq.x.
11. A composition as claimed in claim 6 in which x.ltoreq.1.3.
12. A composition as claimed in claim 6 in which z2.ltoreq.0.3.
13. A composition having the formula
A.sub.xM.sub.yM.sup.i.sub.ziO.sub.2-d, where A is sodium or a mixed
alkali metal including sodium as a major constituent;
0.5.ltoreq.x.ltoreq.1; M is a transition metal not including
nickel; y>0; M.sup.i.sub.zi, for i=1, 2, 3 . . . n is
M.sup.1.sub.z1M.sup.2.sub.z2M.sup.3.sub.z3 . . . M.sup.n.sub.zn,
and wherein each of M.sup.1.sub.z1M.sup.2.sub.z2M.sup.3.sub.z3 . .
. M.sup.n.sub.zn is germanium or a metal not including nickel;
zi.gtoreq.0 for each i=1, 2, 3 . . . n; 0<d.ltoreq.0.5; the
values of x, y, zi for i=1, 2, 3 . . . n, and d are such as to
maintain charge neutrality; and the values of y, zi for i=1, 2, 3 .
. . n, and d are such that y+.SIGMA.zi>1/2(2-d).
14. A composition as claimed in claim 13 wherein M.sup.i, for i=1,
2, 3 . . . n, is selected from the group consisting of: nickel,
iron, cobalt, manganese, titanium, aluminium, magnesium, calcium
and zirconium.
15. A composition as claimed in claim 1 in which A is a mixed
alkali metal including sodium and lithium.
16. A composition as claimed in claim 6 in which A is a mixed
alkali metal including sodium and lithium.
17. A composition as claimed in claim 13 in which A is a mixed
alkali metal including sodium and lithium.
18. A composition as claimed in claim 1 in which A is sodium.
19. A composition as claimed in claim 6 in which A is sodium.
20. A composition as claimed in claim 13 in which A is sodium.
Description
TECHNICAL FIELD
The present invention related to electrodes that contain an active
material comprising Layered oxide materials which show novel anion
stoichiometry, and to the use of such electrodes, for example in a
sodium ion battery application. The invention is also related to
certain novel materials and to the use of these materials for
example as an electrode material.
BACKGROUND ART
The lithium-ion rechargeable battery technology has received a lot
of attention in recent years and is used in most electronic devices
today. Lithium is not a cheap or abundant metal and it is
considered to be too expensive to some markets requiring
rechargeable battery technologies; such as large scale stationary
energy storage applications. Sodium-ion batteries are similar to
lithium ion batteries, both are rechargeable and comprise an anode
(negative electrode), a cathode (positive electrode) and an
electrolyte material, both are capable of storing energy, and they
both charge and discharge via a similar reaction mechanism. When a
sodium-ion (or lithium-ion battery) is charging, Na.sup.+ (or
Li.sup.+) ions de-intercalate from the cathode and insert into the
anode. Concurrently charge balancing electrons pass from the
cathode through an external circuit containing the charger and into
the anode of the battery. During discharge the same process occurs
but in the opposite direction.
Sodium ion battery technology is considered to offer certain
advantages over lithium. Sodium is more abundant than lithium and
some researchers think that this will offer a solution for a low
cost and durable energy storage requirement, particularly for large
scale applications such as grid level energy storage. Nevertheless
a significant amount of development is required before sodium-ion
batteries are a commercial reality.
There are a number of material types which have been shown to be
useful in rechargeable sodium ion batteries which include;
Metallate materials, Layered oxide materials, polyanionic
compounds, phosphates and silicates. However, one of the most
attractive classes of material is that of the layered oxides.
A well-known layered oxide material has the formula
NaNi.sub.0.5Mn.sub.0.5O.sub.2. In this material the transition
metal nickel is present as Ni.sup.2+ while the manganese is present
as Mn.sup.4+. This is an ordered material with the Na and Ni atoms
residing in discrete sites within the structure. In this case the
nickel ions (Ni.sup.2+) are a redox active element which
contributes to the reversible specific capacity and the manganese
ions (Mn.sup.4+) play the role of a structure stabilizer.
Similarly, the compound NaNi.sub.0.5Ti.sub.0.5O.sub.2 is analogous
to NaNi.sub.0.5Mn.sub.0.5O.sub.2 in that the Ni.sup.2+ ions provide
the active redox centre and the Ti.sup.4+ ions are present for
structure stabilization. There is a considerable quantity of
literature describing NaNi.sub.0.5Mn.sub.0.5O.sub.2 and the
titanium analogue, as a precursor for the lithium layered oxide
material LiNi.sub.0.5Mn.sub.0.5O.sub.2 and
NaNi.sub.0.5Ti.sub.0.5O.sub.2 and the subsequent ion exchanging the
sodium for lithium. However, recent electrochemical studies
reported by Komaba et al Adv. Funct. Mater. 2011, 21, 3859 describe
the sodium insertion performance of hard-carbon and layered
NaNi.sub.0.5Mn.sub.0.5O.sub.2 electrodes in propylene carbonate
electrolyte solutions. The results obtained show that
NaNi.sub.0.5-Mn.sub.0.5O.sub.2 exhibits some reversible charging
and discharging ability, unfortunately however the capacity of the
material fades by 25% or more, after only 40 cycles which makes the
use of this material extremely disadvantageous for rechargeable
energy storage applications. As such there is significant interest
in improving the electrochemical performance of such materials.
This invention discloses an oxygen non-stoichiometric material
compositions based on a layered oxide framework. Herein, we define
oxygen non-stoichiometry as deviation from the ABO.sub.2 formula of
the layered oxide framework, wherein in a pristine material the
ratio of elements is 1:1:2. Within this invention we claim that the
ratio deviates from the ideal ABO.sub.2 stoichiometry in the
following manner ABO.sub.2-.delta. wherein, the average oxidation
state of one or more elements contained within the B site reduces
to rebalance the structures charge and retain charge neutrality
whilst the proportion of elements on the B site remains
unchanged.
This yields the same atomic ratios within the A and B site only
with variation on the O site. So the relative proportions of
elements within the material can be expressed as 1:1:2.sub.-.delta.
and the average oxidation state of each site can be expressed as
+1:+3.sub.-2/.delta.:-4.sub.-.delta.
Most of the background literature for these types of materials is
based upon stoichiometric sodium transition metal oxides which
adopt either a P2 or O3 layered structure. This invention describes
novel compositions based on the sodium layered oxides for
application in a sodium ion battery. Within this material class
there is substantial prior art based on material composition,
focused on the Na content and ratios of transition metal
elements.
For example, US20070218361 A1 describes a material suitable for a
sodium ion battery and claim a positive electrode based on a
sodium-containing transition metal oxide. The compositions of the
oxide is described as NaaLibMn.sub.xMy0.sub.2.+-.c, where, M may
include at least one selected from the group consisting of iron,
cobalt, and nickel, a is in the range 0.6 to 1.1 and b may range
from 0 to 0.5, the sum of x and y may range from 0.9 to 1.1, and c
may be from 0 to 0.1. In this application it is claimed that the
number of oxygen atoms in the materials formula unit is based on
charge compensation of the structure. In this compositional claim
the O content is linked to the Na/Li content of the material and
cannot be deficient i.e. there cannot be fewer oxygen atoms in the
material than the sum of alkali and transition metals.
Similarly, in U.S. Pat. No. 8,835,041B2 a layered oxide material
suitable for application in an energy storage device is defined.
The compositional claims of this patent are described by the
formula Na.sub.cLi.sub.dNi.sub.eMn.sub.fM.sub.z0.sub.b, wherein M
comprises one or more metal cations, and the ratios of constituents
are limited to 0.24<c/b=<0.5, 0<d/b=<0.23,
0=<e/b=<0.45, 0=<f/b=<0.45, 0=<z/b=<0.45, the
combined average oxidation state of the metal components is in the
range of about 3.9 to 5.2, and b is equal to (c+d+Ve+Xf+Yz)/2,
wherein V is the average oxidation state of the Ni, X is the
average oxidation state of the Mn, and Y is the average oxidation
state of the M in the material. M may be selected from Mg2+, Co2+,
Co3+, B3+, Fe2+, Fe3+, Ga3+, A13+, and Ti4+.
Further to these disclosures, JP2012252962A claims a positive
electrode for a secondary battery, expressed by the formula
Li.sub.aA.sub.bM.sub.cO.sub.d, where A represents one or more
elements selected from the group consisting of Na and K, M
represents one or more transition metal elements,
0<a.ltoreq.1.5, 0.ltoreq.b<1.5, 0<c.ltoreq.3,
0<d.ltoreq.6, and 0<a+b.ltoreq.1.5. The patent refers to
these materials adopting a spinel type structure rather than a
layered oxide structure.
Further to these documents,
U.S. Pat. No. 8,709,655B2, JP5085032B2, US20090159838A1,
WO2006/057307, JP2006179473A, US20110003192A1, and WO2009/099061
detail the application of sodium layered oxides for application in
energy storage devices The compositional claims of these documents
limit the content of oxygen in the material to O2 and they do not
contain any oxygen non stoichiometry.
SUMMARY OF INVENTION
To the accomplishment of the foregoing and related ends, the
invention, comprises the features hereinafter fully described and
identified in the claims. The following description and the annexed
drawings set forth detail certain illustrative embodiments of the
invention. These embodiments are indicative, however, of but a few
of the various ways in which the principles of the invention may be
employed. Other objects, advantages and novel features of the
invention will become apparent from the following detailed
description of the invention when considered in conjunction with
the drawings.
A first aspect of the invention provides a composition having the
formula A.sub.xM.sub.yM.sup.i.sub.ziO.sub.2-d,
where:
A is sodium or a mixed alkali metal including sodium as a major
constituent;
x>0.5;
M is a transition metal;
y>0;
M.sup.i, for i=1, 2, 3 . . . n, is a metal or germanium;
z.sub.1>0;
z.sub.i.gtoreq.0 for each i=2, 3 . . . n;
0<d.ltoreq.0.5;
the values of x, y, z.sub.i and d are such as to maintain charge
neutrality; and
the values of y, z.sub.i and d are such that
y+.SIGMA.z.sub.i>1/2(2-d).
The present invention provides new compositions that display oxygen
non-stoichiometry. The concept of oxygen non-stoichiometry will be
briefly explained with regard to the composition ABO.sub.2 in which
element(s) A occupy one site and has an oxidation state of +1 and
in which element(s) B occupy another site and has an oxidation
state of +3. In an ideal (stoichiometric) material, the ratio of
elements is A:B:O=1:1:2, and the oxygen has an oxidation state of
-2 giving charge neutrality.
The ratio of elements may deviate from the ideal ABO.sub.2
stoichiometry to give a composition with the non-stoichiometric
formula ABO.sub.2-.delta., with the average oxidation state of one
or more elements contained within the B site being less than 3 to
retain charge neutrality. However, the proportion of elements on
the B site does not change, and there is variation only on the O
site. So the relative proportions of elements within the material
can be expressed as 1:1:(2-.delta.) and the composition may be
thought of as "oxygen deficient" in that the oxygen content of
(2-.delta.) is below the "ideal" value of 2. In a similar manner,
it is in principle possible to obtain an "oxygen rich" composition
with the formula ABO.sub.2+.delta. with the average oxidation state
of one or more elements contained within the B site being above 3
to retain charge neutrality.
The term "transition metal" as used herein includes the f-block
lanthanides and actinides (sometimes referred to as the "inner
transition metals") as well as groups 3 to 12 of the periodic
table.
The composition may adopt a layered oxide structure in which the
alkali metal atoms are co-ordinated by oxygen in a prismatic
environment, or it may adopt a layered oxide structure in which the
alkali metal atoms are co-ordinated by oxygen in an octahedral
environment.
The values of x, y, z.sub.i and d may be such that
x+y+.SIGMA.z.sub.i>2-d.
M.sup.i, for i=1, 2, 3 . . . n, may be a transition metal, an
alkali metal or an alkaline earth metal.
M.sup.i, for i=1, 2, 3 . . . n, may be selected from the group
consisting of: nickel, iron, cobalt, manganese, titanium,
aluminium, magnesium and zirconium. However, the invention is not
limited to this and M.sup.i may alternatively be selected from the
group consisting of: sodium, lithium, potassium, nickel, iron,
cobalt, manganese, titanium, vanadium, niobium, tantalum, hafnium,
chromium, molybdenum, tungsten, osmium, palladium, platinum,
copper, silver, gold, zinc, cadmium, aluminium, scandium, yttrium,
zirconium, technetium, rhenium, ruthenium, rhodium, iridium,
mercury, gallium, indium, lead, bismuth, selenium, magnesium,
calcium, beryllium, strontium, barium, boron, germanium, arsenic,
antimony and tellurium.
Optionally 0.5.ltoreq.a.ltoreq.0.67. Examples of such compositions
include
Na.sub.0.67 Ni.sub.0.3333Mn.sub.0.6666O.sub.2-d
Na.sub.0.67 Ni.sub.0.3333Mn.sub.0.6666-xTi.sub.xO.sub.2-d
Na.sub.0.67 Ni.sub.0.3333Mn.sub.0.6666-xZr.sub.xO.sub.2-d
Na.sub.0.67 Ni.sub.0.3333-x/2Mn.sub.0.6666-x/2Fe.sub.xO.sub.2-d
Na.sub.0.67 Ni.sub.0.3333-x/2Mn.sub.0.6666-x/2Co.sub.xO.sub.2-d
Na.sub.0.67 Ni.sub.0.3333-x/2Mn.sub.0.6666-x/2Al.sub.xO.sub.2-d
Na.sub.0.67 Ni.sub.0.3333-x/2Ti.sub.0.6666-x/2Fe.sub.xO.sub.2-d
Na.sub.0.67Ni.sub.0.3333-x/2Ti.sub.0.6666-x/2Co.sub.xO.sub.2-d
Na.sub.0.67Ni.sub.0.3333-x/2Ti.sub.0.6666-x/2Al.sub.xO.sub.2-d
Na.sub.0.67Ni.sub.0.3333-x/2Ti.sub.0.6666-y-(x/2)Mn.sub.yCo.sub.xO.sub.2--
d
Na.sub.0.67Ni.sub.0.3333-x/2Ti.sub.0.6666-y-(x/2)Mn.sub.yFe.sub.xO.sub.2--
d
Na.sub.0.67
Ni.sub.0.3333-x/2Ti.sub.0.6666-y-(x/2)Mn.sub.xAl.sub.xO.sub.2-d
Optionally 0.67.ltoreq.a.ltoreq.0.76. Examples of such compositions
include
Na.sub.0.76Mn.sub.0.65-(x+y)Co.sub.yNi.sub.xO.sub.2.sub.-d
Na.sub.0.76Mn.sub.0.65-(x+y)Fe.sub.yNi.sub.xO.sub.2.sub.-d
Na.sub.0.76Mn.sub.0.65-((x-z)+y)Co.sub.yNi.sub.x-zMg.sub.zO.sub.2.sub.-d
Na.sub.0.76Mn.sub.0.65-((x-z)+y)Fe.sub.yNi.sub.x-zMg.sub.zO.sub.2.sub.-d
Na.sub.0.76Mn.sub.0.65-((X-z)+y)Co.sub.yFe.sub.x-z
Mg.sub.zO.sub.2.sub.-d
Na.sub.0.76Ti.sub.0.65-(x+y)Co.sub.yNi.sub.xO.sub.2.sub.-d
Na.sub.0.76Ti.sub.0.65-(x+y)Fe.sub.yNi.sub.xO.sub.2.sub.-d
Na.sub.0.76Ti.sub.0.65-((x-z)+y)Co.sub.yNi.sub.x-zMg.sub.zO.sub.2.sub.-d
Na.sub.0.76Ti.sub.0.65-((x-z)+y)Fe.sub.yNi.sub.x-zMg.sub.zO.sub.2.sub.-d
Na.sub.0.76Ti.sub.0.65-((x-z)+y)Co.sub.yFe.sub.x-zMg.sub.zO.sub.2.sub.-d
Na.sub.0.76Ti.sub.aMn.sub.0.65-a-((x-z)+y)CO.sub.yFe.sub.x-zMg.sub.zO.sub-
.2.sub.-d
A second aspect of the invention provides a composition having the
formula A.sub.xM.sub.yM.sup.1.sub.z1M.sup.i.sub.ziO.sub.2-d,
where
A is sodium or a mixed alkali metal including sodium as a major
constituent;
x>0;
M is a transition metal;
y>0;
M.sup.1 is an alkali metal or a mixed alkali metal;
z.sub.1>0;
M.sup.i, for i=2, 3, 4 . . . n, is a metal or germanium;
z.sub.2>0;
z.sub.i.gtoreq.0 for each i=3, 4 . . . n;
0<d.ltoreq.0.5;
the values of x, y, z.sub.i and d are such as to maintain charge
neutrality;
the values of y, z.sub.i and d are such that
y+.SIGMA.z.sub.i>1/2(2-d); and
the composition adopts a layered oxide structure in which the
alkali metal atoms A are co-ordinated by oxygen in a prismatic or
octahedral environment, and in which the alkali metal atoms M.sup.1
adopts a co-ordination complementary to that adopted by the other
Mi transition metal element(s).
In general these materials have a structure with only one
transition metal site, unless there is ordering. It is believed
that the further alkali metal (or mixed alkali metal) atoms M.sup.1
occupy the transition metal site together with the transition metal
atoms M and the additional metal constituent M.sup.2 (and any other
further constituent M.sup.i that might be present), and so adopt a
co-ordination that is complementary to the other metal constituents
present. The sodium (or mixed alkali metal including sodium as a
major constituent) atoms A are believed to occupy the alkali metal
site in the material structure.
In a composition of this aspect, the further alkali metal
constituent M.sup.1 may be the same as the first alkali metal
constituent A (for example, Na Na.sub.0.167
Ni.sub.0.25Mn.sub.0.5833 O.sub.2-d (or Na
Na.sub.1/6Mn.sub.7/12-xTi.sub.xO.sub.2-d)) or it may be different
to the first alkali metal constituent A (for example
NaLi.sub.0.2Ni.sub.0.25Mn.sub.0.55O.sub.2-d). Accordingly, some
compositions according to this aspect may be written with formulae
of the type "Na Na.sub.0.167 Ni.sub.0.25Mn.sub.0.5833O.sub.2-d", in
which an alkali metal (in this example sodium) is written twice to
indicate that ions are on two distinct sites in the material
structure. In this example the first occurrence ("Na") denotes
sodium ions occupying the alkali metal site of the material
structure, and the second occurrence ("Na.sub.0.167") denotes
sodium ions believed to occupy the transition metal site of the
material structure.
It should also be noted that some compositions according to this
aspect may have formulae of the type "Na Na.sub.0.167-y Li.sub.y
Ni.sub.0.25Mn.sub.0.5833O.sub.2-d". In such a composition the
"Na.sub.0.167-y Li.sub.y" denotes a mixed alkali metal that
occupies the transition metal site, and the initial "Na" again
denotes sodium ions occupying the alkali metal site.
The values of x, y, z.sub.i and d may be such that
x+y+.SIGMA.z.sub.i>2-d.
M.sup.i, for i=3, 4 . . . n, may be a transition metal, an alkali
metal or an alkaline earth metal.
M.sup.1 may be selected from the group consisting of: sodium,
lithium, and a mixture of sodium and lithium.
M.sup.i, for i=2, 3 . . . n, may be selected from the group
consisting of: nickel, iron, cobalt, manganese, titanium,
aluminium, magnesium and zirconium. However, the invention is not
limited to this and M.sup.i may alternatively be selected from the
group consisting of: sodium, lithium, potassium, nickel, iron,
cobalt, manganese, titanium, vanadium, niobium, tantalum, hafnium,
chromium, molybdenum, tungsten, osmium, palladium, platinum,
copper, silver, gold, zinc, cadmium, aluminium, scandium, yttrium,
zirconium, technetium, rhenium, ruthenium, rhodium, iridium,
mercury, gallium, indium, lead, bismuth, selenium, magnesium,
calcium, beryllium, strontium, barium, boron, germanium, arsenic,
antimony and tellurium.
Optionally 0.9.ltoreq.x.
Optionally x.ltoreq.1.3.
Optionally z.sub.2.ltoreq.0.3.
NaLi.sub.0.2Ni.sub.0.25Mn.sub.0.55O.sub.2-d
NaLi.sub.0.2Ni.sub.0.25Mn.sub.0.55-xTi.sub.xO.sub.2-d
NaLi.sub.0.2Ni.sub.0.25-x/2Mn.sub.0.55-x/2Fe.sub.xO.sub.2-d
NaLi.sub.0.2Ni.sub.0.25-x/2Mn.sub.0.55-x/2Co.sub.xO.sub.2-d
NaLi.sub.0.2Ni.sub.0.25-x/2Mn.sub.0.55-x/2Al.sub.xO.sub.2-d
NaLi.sub.0.2Ni.sub.0.25-xMn.sub.0.55-y-(x/2)Ti.sub.y-x/2Fe.sub.xO.sub.2-d
NaLi.sub.0.2Ni.sub.0.25-xMn.sub.0.55-y-(x/2)Ti.sub.y-x/2Co.sub.xO.sub.2-d
NaLi.sub.0.2Ni.sub.0.25-xMn.sub.0.55-y-(x/2)Ti.sub.y-x/2Al.sub.xO.sub.2-d
Na.sub.0.9Li.sub.0.3Ni.sub.0.25Mn.sub.0.55O.sub.2-d
Na.sub.0.9Li.sub.0.3Ni.sub.0.25Mn.sub.0.55-xTi.sub.xO.sub.2-d
Na.sub.0.9Li.sub.0.3Ni.sub.0.25-xMn.sub.0.55-xFe.sub.xO.sub.2-d
Na.sub.0.9Li.sub.0.3Ni.sub.0.25-xMn.sub.0.55-xCo.sub.xO.sub.2-d
Na.sub.0.9Li.sub.0.3Ni.sub.0.25-xMn.sub.0.55-xAl.sub.xO.sub.2-d
Na.sub.0.9Li.sub.0.3Ni.sub.0.25-xMn.sub.0.55-y-(x/2)Ti.sub.y-x/2Fe.sub.xO-
.sub.2-d
Na.sub.0.9Li.sub.0.3Ni.sub.0.25-xMn.sub.0.55-y-(x/2)Ti.sub.y-x/2Co.sub.xO-
.sub.2-d
Na.sub.0.9Li.sub.0.3Ni.sub.0.25-xMn.sub.0.55-y-(x/2)Ti.sub.y-x/2Al.sub.xO-
.sub.2-d
Na Na.sub.0.167 Ni.sub.0.25Mn.sub.0.5833O.sub.2-d
Na Na.sub.0.167 Ni.sub.0.25Mn.sub.0.5833-xTi.sub.xO.sub.2-d
Na Na.sub.0.167 Ni.sub.0.25Mn.sub.0.5833-xZr.sub.xO.sub.2-d
Na Na.sub.0.167-y Li.sub.yNi.sub.0.25Mn.sub.0.5833O.sub.2-d
Na Na.sub.0.167-y
Li.sub.yNi.sub.0.25Mn.sub.0.5833Ti.sub.xO.sub.2-d
Na Na.sub.0.167-y
Li.sub.yNi.sub.0.25Mn.sub.0.5833Zr.sub.xO.sub.2-d
Na Na.sub.0.167-y
Li.sub.yNi.sub.0.25-x/2Mn.sub.0.5833-x/2Fe.sub.xO.sub.2-d
Na Na.sub.0.167-y
Li.sub.yNi.sub.0.25-x/2Mn.sub.0.5833-x/2Co.sub.xO.sub.2-d
Na Na.sub.0.167-y
Li.sub.yNi.sub.0.25-x/2Mn.sub.0.5833-x/2Al.sub.xO.sub.2-d
It should be noted that in compositions such as
Na.sub.0.9Li.sub.0.3Ni.sub.0.25Mn.sub.0.55O.sub.2-d above, some of
the Li atoms form a mixed alkali metal with the Na atoms which is
believed to occupy the alkali metal site, and some of the Li atoms
are the further alkali metal constituent M.sup.1. This composition
may therefore alternatively be thought of as:
(Na.sub.0.9Li.sub.0.1)Li.sub.0.2Ni.sub.0.25Mn.sub.0.55O.sub.2-d,
and similarly for the other compositions of this type.
A third aspect of the invention provides a composition having the
formula A.sub.xM.sub.yM.sup.i.sub.ziO.sub.2-d, where
A is an sodium or a mixed alkali metal including sodium as a major
constituent;
x>0;
M is a transition metal not including nickel;
y>0;
M.sup.i, for i=1, 2, 3 . . . n, is germanium or a metal not
including nickel;
z.sub.i.gtoreq.0 for each i=1, 2, 3 . . . n;
0<d.ltoreq.0.5; and
the values of x, y, z.sub.i and d are such as to maintain charge
neutrality.
The values of y, z.sub.i and d may be such that
y+.SIGMA.z.sub.i>1/2(2-d)
The values of x, y, z.sub.i and d may be such that
x+y+.SIGMA.z.sub.i>2-d.
M.sup.i, for i=1, 2, 3 . . . n, may be selected from the group
consisting of: iron, cobalt, manganese, titanium, aluminium,
magnesium, calcium and zirconium. However, the invention is not
limited to this and M.sup.i may alternatively be selected from the
group consisting of: sodium, lithium, potassium, nickel, iron,
cobalt, manganese, titanium, vanadium, niobium, tantalum, hafnium,
chromium, molybdenum, tungsten, osmium, palladium, platinum,
copper, silver, gold, zinc, cadmium, aluminium, scandium, yttrium,
zirconium, technetium, rhenium, ruthenium, rhodium, iridium,
mercury, gallium, indium, lead, bismuth, selenium, magnesium,
calcium, beryllium, strontium, barium, boron, germanium, arsenic,
antimony and tellurium.
Examples of compositions according to this aspect include
compositions assuming Co is in a +3 oxidation state, and having the
formula: NaMg.sub.xMn.sub.yCo.sub.1-(x+y)O.sub.2-.delta.(where
x=y).
Other compositions according to this aspect include compositions
assuming Fe is in a +3 oxidation state, and having the formula:
NaMg.sub.xMn.sub.yFe.sub.1-(x+y)O.sub.2-.delta.(where x=y).
A fourth aspect of the invention provides an electrode comprising
composition according to one of the first to third aspects. The
composition may form an active element of the electrode. An
electrode of this aspect may be used in conjunction with a counter
electrode and one or more electrolyte materials. The electrolyte
material may comprise an aqueous electrolyte material, or it may
comprise a non-aqueous electrolyte.
A fifth aspect of the invention provides an energy storage device
comprising an electrode of the fourth aspect.
The energy storage device may be suitable for use as one or more of
the following: a sodium and/or potassium ion cell; a sodium and/or
potassium metal cell; a non-aqueous electrolyte sodium and/or
potassium ion cell; and an aqueous electrolyte sodium and/or
lithium and/or potassium ion cell.
A sixth aspect of the invention provides a rechargeable battery
comprising at least one of an electrode of the fourth aspect and an
energy storage device of the fifth aspect.
A seventh aspect of the invention provides an electrochemical
device comprising at least one of an electrode of the fourth aspect
and an energy storage device of the fifth aspect.
A eighth aspect of the invention provides an electrochromic device
comprising at least one of an electrode of the fourth aspect and an
energy storage device of the fifth aspect.
A ninth aspect of the invention provides an oxide ion conductor
comprising a material having a composition according to one of the
first to third aspects.
A composition of the invention may be prepared by mixing starting
materials together, heating the mixed starting materials at a
temperature of between 400.degree. C. and 1500.degree. C., to
obtain an oxygen-deficient reaction product, and cooling the
reaction product, or allowing the reaction product to cool, under
conditions that prevent significant re-incorporation of oxygen into
the oxygen-deficient reaction product.
Since the reaction product is cooled, or allowed to cool, under
conditions that prevent significant re-incorporation of oxygen into
the oxygen-deficient reaction product, the end product is an oxygen
deficient composition having the formula A.sub.x M.sub.y
M.sup.i.sub.ziO.sub.2-d. (It should be noted that some slight
re-incorporation of oxygen may occur, so that the oxygen fraction
2-d of the end product may not be exactly equal to the oxygen
fraction of the oxygen-deficient reaction product obtained by
heating the starting materials.)
The present invention can provides a bulk material with an oxygen
deficient composition. In contrast to some prior techniques can
only provide mild levels of reduction (that is, mild levels of
oxygen loss) on the surface of the sample, and cannot provide a
bulk oxygen deficient material.
The mixed starting materials may be heated for more than 30 seconds
and/or may be heated for less than 200 hours, and optionally may be
heated for more than 2 hours and/or may be heated for less than 200
hours.
Cooling the reaction product may comprise cooling the reaction
product in one of an inert atmosphere and a reducing
atmosphere.
Cooling the reaction product may comprise cooling the reaction
product in an inert atmosphere, the inert atmosphere being, or
consisting substantially of, one or more inert gases. Examples of
suitable inert gases include nitrogen, and argon and other of the
noble gases.
Alternatively, cooling the reaction product may comprise cooling
the reaction product in a reducing atmosphere such as, for example,
an atmosphere of hydrogen in nitrogen. However, the invention does
not require cooling the reaction product in a reducing atmosphere,
and requires only that the cooling is carried out under conditions
that prevent significant re-incorporation of oxygen.
Heating the mixed starting materials to obtain an oxygen-deficient
reaction product may comprise heating the mixed starting materials
in an oxidising atmosphere.
Heating the mixed starting materials to obtain an oxygen-deficient
reaction product may comprise heating the mixed starting materials
in air.
As set out above, the present invention provides compounds of the
formula: A.sub.uM.sub.1vO.sub.2-d
Wherein,
A comprises one or more alkali metals or combination thereof
(lithium, sodium and potassium)
M.sub.1 can be a transition metal or mixture thereof in suitable
proportions to maintain charge neutrality in the composition and
may consist of metals in oxidation states of +1, +2, +3 or +4 in
any ratio or combination.
U may range from 0.5<U<2
V may range from 0.25<V<1.5; 0<d<0.5
The above formula includes compounds that are oxygen deficient.
Further the oxidation states may or may not be integers i.e. they
may be whole numbers or fractions or a combination of whole numbers
and fractions and may be averaged over different crystallographic
sites in the material.
The present Applicant has found that not only are the oxidation
states of the metal constituents in the compounds of the present
invention a critical feature to the production of highly
electrochemically active compounds but they have also confirmed
that having particular transition metal constituents allows
variable oxidation states (i.e. oxidation states which are not
integers) in the same crystalline structure of the compound. It is
known that that there are several possible layered structural forms
which alkali metal/metal/oxides may adopt, including O3, P3 and P2.
The Applicant has shown that the oxidation states for the metal
constituents can allow oxidation states which are not integers to
be stabilised in many structural forms including O3, P3 and P2 via
the loss of oxygen from the material. The magnitude of the loss of
oxygen may also be controlled by tailoring the synthesis of these
materials. The applicant has also noted several notable benefits in
the application of these materials in electrochemical devices.
Materials which show oxygen deficiency generally show lower
irreversibility on alkali metal intercalation and de intercalation,
they also show similar capacity retention and similar intercalation
potentials. The slightly lower oxygen content also results in
dilation of the unit cell and realises a smaller volume change on
electrochemical cycling, leading to improvements in cell
lifetime.
Preferred compounds of the present invention include:
e.g. Na.sub.0.67Ni.sub.0.333Mn.sub.0.666O.sub.2-d
Na.sub.0.76Mn.sub.0.65Co.sub.0.18Ni.sub.0.17O.sub.2-d
NaLi.sub.0.2Ni.sub.0.25Mn.sub.0.75O.sub.2-d
Na.sub.0.9Li.sub.0.3Ni.sub.0.25Mn.sub.0.75O.sub.2-d
NaNi.sub.0.25Li.sub.0.166Mn.sub.7/12O.sub.2-d
Na Ni.sub.0.25Na.sub.0.17Ti.sub.7/12O.sub.2.sub.-d
Na Ni.sub.0.25Na.sub.0.17Mn.sub.2/12Ti.sub.5/12O.sub.2.sub.-d
Na Ni.sub.0.25Na.sub.0.17Mn.sub.5/12Ti.sub.2/12O.sub.2.sub.-d
Na.sub.2/3Ni.sub.1/3Mn.sub.2/3O.sub.2.sub.-d
Na.sub.0.76Mn.sub.0.65CO.sub.0.18Ni.sub.0.17O.sub.2.sub.-d
The magnitude of d can be controlled.
Additionally preferred electrodes of the present invention comprise
active compounds defined above in which d>0.01.
It is particularly advantageous if the electrode comprises active
compounds in which transition metals of oxidation states +1, +2 and
+4 are combined on the metal site.
The presence of oxygen deficiency is particularly advantageous to
improve the electrochemical irreversibility on cycling; resulting
in the active materials which are capable of being charged and
recharged numerous times with a lower drop in capacity on the first
electrochemical cycle. The active materials comprising oxygen
deficiency are also advantageous because the reduction of Oxygen
content does not appear to have any adverse effect on the
intercalation potentials of the material, but yields a few percent
gain in gravimetric energy density.
Preferred electrodes of the present invention comprise active
compounds selected from one or more of:
Na.sub.0.67Ni.sub.0.333Mn.sub.0.666O.sub.2-d
NaLi.sub.0.2Ni.sub.0.25Mn.sub.0.75O.sub.2-d
Na.sub.0.9Li.sub.0.3Ni.sub.0.25Mn.sub.0.75O.sub.2-d
NaNi.sub.0.25Li.sub.0.166Mn.sub.7/12O.sub.2-d
Na Ni.sub.0.25Na.sub.0.17Ti.sub.7/12O.sub.2-d
Na Ni.sub.0.25Na.sub.0.17Mn.sub.2/12Ti.sub.5/12O.sub.2.sub.-d
Na Ni.sub.0.25Na.sub.0.17Mn.sub.5/12Ti.sub.2/12O.sub.2.sub.-d
Na.sub.2/3Ni.sub.1/3Mn.sub.2/3O.sub.2.sub.-d
Na.sub.0.76Mn.sub.0.65CO.sub.0.18Ni.sub.0.17O.sub.2.sub.-d
The electrodes according to the present invention are suitable for
use in many different applications, for example energy storage
devices, rechargeable batteries, electrochemical devices and
electrochromic devices.
Advantageously, the electrodes according to the invention are used
in conjunction with a counter electrode and one or more electrolyte
materials. The electrolyte materials may be any conventional or
known materials and may comprise either aqueous electrolyte(s) or
non-aqueous electrolyte(s) or mixtures thereof.
In a further aspect, the present invention provides an energy
storage device that utilises an electrode comprising the active
materials described above, and particularly an energy storage
device for use as one or more of the following: a sodium and/or
lithium and/or potassium ion cell; a sodium and/or lithium and/or
potassium metal cell; a non-aqueous electrolyte sodium and/or
potassium ion; an aqueous electrolyte sodium and/or lithium and/or
potassium ion cell.
The novel compounds of the present invention may be prepared using
any known and/or convenient method. For example, the precursor
materials may be heated in a furnace so as to facilitate a solid
state reaction process. They may be further processed at elevated
temperatures to induce, reduce or increase the magnitude of oxygen
deficiency in the crystal structure.
A particularly advantageous method for the preparation of the
compounds described above comprises the steps of:
a) mixing the starting materials together, preferably intimately
mixing the starting materials together and further preferably
pressing the mixed starting materials into a pellet;
b) heating the mixed starting materials in a furnace at a
temperature of between 400.degree. C. and 1500.degree. C.,
preferably a temperature of between 500.degree. C. and 1200.degree.
C., for between 2 and 200 hours. Heating may be undertaken using; a
static atmosphere, a flowing gas or a purging gas. Precursor
materials may be heated under, oxygen containing atmospheres,
reducing atmospheres or inert atmospheres; and
c) allowing the reaction product to cool. Cooling may also be
undertaken using; a static atmosphere, a flowing gas or a purging
gas. Precursor materials may be cooled under, oxygen containing
atmospheres, reducing atmospheres or inert atmospheres.
Preferably the reaction is conducted under an atmosphere of ambient
air, and alternatively under an inert gas.
DESCRIPTION OF DRAWINGS
FIG. 1(A) shows Powder X-ray diffraction pattern of
NaNi.sub.0.25Na.sub.0.17Mn.sub.4/12Ti.sub.3/12O.sub.2 prepared
according to Example 1;
FIG. 1(B) shows the TGA-STA data obtained for
NaNi.sub.0.25Na.sub.0.17Mn.sub.4/12Ti.sub.3/12O.sub.2 (Example 1)
heated to a temperature of 800.degree. C. under a constant flow of
air (dashed line) or under a constant flow of nitrogen (solid
line).
FIG. 1(C) Powder X-ray diffraction patterns of
NaNi.sub.0.25Na.sub.0.17Mn.sub.4/12Ti.sub.3/12O.sub.2 after being
heated to 800.degree. C. and cooled in either air or nitrogen.
FIG. 1(D) shows the first three charge-discharge voltage profiles
(Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific
capacity [mAh/g]) for the cathode material prepared according to
Example 1 after heating to 800.degree. C. under a constant flow of
air (dashed line) or under a constant flow of nitrogen (solid
line).
FIG. 1(E) shows the Differential Capacity Profiles for the 1.sup.st
charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half
Cell Voltage [V vs Na/Na+]) for the cathode material prepared
according to Example 1 heated to a temperature of 800.degree. C.
under a constant flow of air (dashed line) or under a constant flow
of nitrogen (solid line).
FIG. 2(A) shows Powder X-ray diffraction pattern of
NaNi.sub.0.25Na.sub.0.17Mn.sub.2/12Ti.sub.5/12O.sub.2 prepared
according to Example 2.
FIG. 2(B) shows the TGA-STA data obtained for
NaNi.sub.0.25Na.sub.0.17Mn.sub.2/12Ti.sub.5/12O.sub.2 (Example 2)
heated to a temperature of 800.degree. C. under a constant flow of
air (dashed line) or under a constant flow of nitrogen (solid
line).
FIG. 2(C) Powder X-ray diffraction patterns of
NaNi.sub.0.25Na.sub.0.17Mn.sub.2/12Ti.sub.5/12O.sub.2 after being
heated to 800.degree. C. and cooled in either air or nitrogen.
FIG. 2(D) shows the first three charge-discharge voltage profiles
(Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific
capacity [mAh/g]) for the cathode material prepared according to
Example 2 after heating to 800.degree. C. under a constant flow of
air (Solid line) or under a constant flow of nitrogen (dashed
line).
FIG. 2(E) shows the cycle life (Cathode specific capacity [mAh/g]
vs cycle number) for the cathode material prepared according to
Example 2 after heating to 800.degree. C. under a constant flow of
air (triangles) or under a constant flow of nitrogen (circles).
Dotted line represents discharge and solid line represents
charge.
FIG. 2(F) shows the Differential Capacity Profiles for the 1.sup.st
charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half
Cell Voltage [V vs Na/Na+]) for the cathode material prepared
according to Example 2 heated to a temperature of 800.degree. C.
under a constant flow of air (solid line) or under a constant flow
of nitrogen (dashed line).
FIG. 3(A) shows Powder X-ray diffraction pattern of
NaNi.sub.0.25Na.sub.0.17Mn.sub.5/12Ti.sub.2/12O.sub.2 prepared
according to Example 3.
FIG. 3(B) shows the TGA-STA data obtained for
NaNi.sub.0.25Na.sub.0.17Mn.sub.5/12Ti.sub.2/12O.sub.2 (Example 3)
heated to a temperature of 800.degree. C. under a constant flow of
air (dashed line) or under a constant flow of nitrogen (solid
line).
FIG. 3(C) Powder X-ray diffraction patterns of
NaNi.sub.0.25Na.sub.0.17Mn.sub.5/12Ti.sub.2/12O.sub.2 after being
heated to 800.degree. C. and cooled in either air or nitrogen.
FIG. 3(D) shows the first four charge-discharge voltage profiles
(Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific
capacity [mAh/g]) for the cathode material prepared according to
Example 3 after heating to 800.degree. C. under a constant flow of
air (solid line) or under a constant flow of nitrogen (dashed
line).
FIG. 3(E) shows the cycle life (Cathode specific capacity [mAh/g]
vs cycle number) for the cathode material prepared according to
Example 3 after heating to 800.degree. C. under a constant flow of
air (triangles) or under a constant flow of nitrogen (circles).
Dotted line represents discharge and solid line represents
charge.
FIG. 3(F) shows the Differential Capacity Profiles for the 1.sup.st
charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half
Cell Voltage [V vs Na/Na+]) for the cathode material prepared
according to Example 3 heated to a temperature of 800.degree. C.
under a constant flow of air (dashed line) or under a constant flow
of nitrogen (solid line).
FIG. 4(A) shows Powder X-ray diffraction pattern of
Na.sub.2/3Ni.sub.1/3Mn.sub.2/3O.sub.2 prepared according to Example
4;
FIG. 4(B) shows the TGA-STA data obtained for
Na.sub.2/3Ni.sub.1/3Mn.sub.2/3O.sub.2 (Example 4) heated to a
temperature of 900.degree. C. under a constant flow of air (dashed
line) or under a constant flow of nitrogen (solid line).
FIG. 4(C) Powder X-ray diffraction patterns of
Na.sub.2/3Ni.sub.1/3Mn.sub.2/3O.sub.2 after being heated to
900.degree. C. and cooled in either air or nitrogen.
FIG. 4(D) shows the first four charge-discharge voltage profiles
(Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific
capacity [mAh/g]) for the cathode material prepared according to
Example 4 after heating to 900.degree. C. under a constant flow of
air (solid line) or under a constant flow of nitrogen (dashed
line).
FIG. 4(E) shows the cycle life (Cathode specific capacity [mAh/g]
vs cycle number) for the cathode material prepared according to
Example 4 after heating to 900.degree. C. under a constant flow of
air (circles) or under a constant flow of nitrogen (triangles).
Dotted line represents discharge and solid line represents
charge.
FIG. 4(F) shows the Differential Capacity Profiles for the 1.sup.st
charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half
Cell Voltage [V vs Na/Na+]) for the cathode material prepared
according to Example 4 heated to a temperature of 900.degree. C.
under a constant flow of air (dashed line) or under a constant flow
of nitrogen (solid line).
FIG. 5(A) shows Powder X-ray diffraction pattern of
Na.sub.0.76Mn.sub.0.65Co.sub.0.18Ni.sub.0.17O.sub.2 prepared
according to Example 5;
FIG. 5(B) shows the TGA-STA data obtained for
Na.sub.0.76Mn.sub.0.65Co.sub.0.18Ni.sub.0.17O.sub.2 (Example 5)
heated to a temperature of 900.degree. C. under a constant flow of
air (dashed line) or under a constant flow of nitrogen (solid
line).
FIG. 5(C) Powder X-ray diffraction patterns of
Na.sub.0.76Mn.sub.0.65Co.sub.0.18Ni.sub.0.17O.sub.2 after being
heated to 900.degree. C. and cooled in either air or nitrogen.
FIG. 5(D) shows the first charge-discharge cycle voltage profile
(Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific
capacity [mAh/g]) for the cathode material prepared according to
Example 5 after heating to 900.degree. C. under a constant flow of
air (solid line) or under a constant flow of nitrogen (dashed
line).
FIG. 5(E) shows the Differential Capacity Profiles for the 1.sup.st
charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half
Cell Voltage [V vs Na/Na+]) for the cathode material prepared
according to Example 5 heated to a temperature of 900.degree. C.
under a constant flow of air (dashed line) or under a constant flow
of nitrogen (solid line).
FIG. 6(A) shows Powder X-ray diffraction pattern of Na
Fe.sub.0.5Ti.sub.0.125Mn.sub.0.125Mg.sub.0.25O.sub.2 prepared
according to Example 5;
FIG. 6(B) shows the TGA-STA data obtained for Na
Fe.sub.0.5Ti.sub.0.125Mn.sub.0.125Mg.sub.0.25O.sub.2 (Example 6)
heated to a temperature of 850.degree. C. under a constant flow of
air (dashed line) or under a constant flow of nitrogen (solid
line).
FIG. 6(C) Powder X-ray diffraction patterns of Na
Fe.sub.0.5Ti.sub.0.125Mn.sub.0.125Mg.sub.0.25 O.sub.2 after being
heated to 850.degree. C. and cooled in either air or nitrogen.
FIG. 6(D) shows the first three charge-discharge voltage profiles
(Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific
capacity [mAh/g]) for the cathode material prepared according to
Example 6 after heating to 850.degree. C. under a constant flow of
air (solid line) or under a constant flow of nitrogen (dashed
line).
FIG. 6(E) shows the cycle life (Cathode specific capacity [mAh/g]
vs cycle number) for the cathode material prepared according to
Example 6 after heating to 850.degree. C. under a constant flow of
air (circles) or under a constant flow of nitrogen (triangles).
Dotted line represents discharge and solid line represents
charge.
FIG. 6(F) shows the Differential Capacity Profiles for the 1.sup.st
charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half
Cell Voltage [V vs Na/Na+]) for the cathode material prepared
according to Example 6 heated to a temperature of 800.degree. C.
under a constant flow of air (dashed line) or under a constant flow
of nitrogen (solid line).
DETAILED DESCRIPTION
The materials according to the present invention are prepared using
the following generic method:
Synthesis Method:
The required amounts of the precursor materials are intimately
mixed together and either pressed into a pellet or retained as a
free flowing powder. The resulting mixture is then heated in a tube
furnace or a chamber furnace using either an ambient air
atmosphere, or a flowing inert atmosphere (e.g. argon or nitrogen),
at a furnace temperature of between 400.degree. C. and 1500.degree.
C. until reaction product forms; for some materials a single
heating step is used and for others (as indicated below in Table 1)
more than one heating step is used. Different cooling protocols can
be used to induce oxygen non-stoichiometry in the materials. Sample
may be heated and cooled under different atmospheres as indicated
in table 1. When cool, the reaction product is removed from the
furnace and ground into a powder prior to characterisation.
Using the above generic method, active materials were prepared,
Examples 1 to 11, as summarised below in Table 1:
TABLE-US-00001 TARGET STARTING Ex. COMPOUND MATERIALS FURNACE
CONDITIONS 1
NaNi.sub.0.25Na.sub.0.17Mn.sub.4/12Ti.sub.3/12O.sub.2-d TiO.sub.2
(d = 0) = 900.degree. C., 8 h, Air Na.sub.2CO.sub.3 (d > 0) =
800.degree. C., 1 h, N.sub.2 NiCO.sub.3 MnCO.sub.3 2
NaNi.sub.0.25Na.sub.0.17Mn.sub.2/12Ti.sub.5/12O.sub.2-d TiO.sub.2
1.sup.- st firing (d = 0) = 900.degree. C., 8 h, Air
Na.sub.2CO.sub.3 2.sup.nd firing (d > 0) = 800.degree. C., 1 h,
N.sub.2 NiCO.sub.3 MnCO.sub.3 3
NaNi.sub.0.25Na.sub.0.17Mn.sub.5/12Ti.sub.2/12O.sub.2-d TiO.sub.2
1.sup.- st firing (d = 0) = 900.degree. C., 8 h, Air
Na.sub.2CO.sub.3 2.sup.nd firing (d > 0) = 800.degree. C., 1 h,
N.sub.2 NiCO.sub.3 MnCO.sub.3 4
Na.sub.2/3Ni.sub.1/3Mn.sub.2/3O.sub.2-d Na.sub.2CO.sub.3 1.sup.st
firing (d = 0) = 900.degree. C., 8 h, Air NiCO.sub.3 2.sup.nd
firing (d > 0) = 800.degree. C., 1 h, N.sub.2 MnCO.sub.3 5
Na.sub.0.76 Mn.sub.0.65Co.sub.0.18Ni.sub.0.17 O.sub.2-d
Na.sub.2CO.sub.3 1.sup.st firing (d = 0) = 900.degree. C., 8 h, Air
TiO.sub.2 2.sup.nd firing (d > 0) = 800.degree. C., 1 h, N.sub.2
NiCO.sub.3 MnCO.sub.3 CoCO.sub.3 6 Na Fe.sub.0.5 Ti.sub.0.125
Mn.sub.0.125Mg.sub.0.25 Na.sub.2CO.sub.3 1.sup.st firing (d = 0) =
900.degree. C., 12 h, Air O.sub.2-d TiO.sub.2 2.sup.nd firing (d
> 0) = 800.degree. C., 1 h, N.sub.2 MgCO.sub.3 Na Fe.sub.0.5
Ti.sub.0.125 Mn.sub.0.125Mg.sub.0.25 O.sub.1.98 MnCO.sub.3
Fe.sub.2O.sub.3 7 Na Fe.sub.0.5 Ti.sub.0.125
Mn.sub.0.125Mg.sub.0.25 Directly synthesised as d > 0 O.sub.1.98
1.sup.st firing 900.degree. C. for 12 h in Air followed by cooling
to room temperature under N.sub.2 or Argon 8
NaNi.sub.0.5Ti.sub.0.5O.sub.2-d TiO.sub.2 Directly synthesised as d
> 0 Na.sub.2CO.sub.3 1.sup.st firing 900.degree. C. for 12 h in
Air NiCO.sub.3 followed by cooling to room temperature under
N.sub.2 or Argon 9
NaNi.sub.0.25Na.sub.0.17Mn.sub.4/12Ti.sub.3/12O.sub.2-d TiO.sub.2
Direct- ly synthesised as d > 0 Na.sub.2CO.sub.3 1.sup.st firing
900.degree. C. for 12 h in Air NiCO.sub.3 followed by cooling to
room MnCO.sub.3 temperature under N.sub.2 or Argon 10
Na.sub.2/3Ni.sub.1/3Mn.sub.2/3O.sub.2-d Na.sub.2CO.sub.3 Directly
synthesised as d > 0 NiCO.sub.3 1.sup.st firing 900.degree. C.
for 12 h in Air MnCO.sub.3 followed by cooling to room temperature
under N.sub.2 or Argon 11 Na Mg.sub.x Mn.sub.x
Co.sub.1-(x+y)O.sub.2-d MgCO.sub.3 Directly synthesised as d > 0
MnCO.sub.3 1.sup.st firing 900.degree. C. for 12 h in Air
CoCO.sub.3 followed by cooling to room Na.sub.2CO.sub.3 temperature
under N.sub.2 or Argon
One example of a method of manufacturing a composition of the
invention is an indirect route in which we first mix the precursors
and then fire the mixture to produce a stoichiometric layered
oxide. To form the oxygen non-stoichiometric form a secondary
processing step is used. The secondary processing step can take one
of two forms, the material may be re heated under air to a
temperature close to the formation temperature of the material and
cooled under a flow of nitrogen. This method relies on preventing
the re-uptake of oxygen in the material. A broadly similar method
can also be used in which the secondary processing step can be
undertake under an inert atmosphere such as an atmosphere
consisting, or consisting substantially of, an inert gas or a
mixture of inert gases. Examples of suitable inert gases include
nitrogen, and argon and the other noble gases. That is, in one
example, both the heating and cooling steps can be conducted under
a nitrogen atmosphere to yield the non-stoichiometric form of the
oxide.
Another example of a method of manufacturing a composition of the
invention is a single step process in which we mix the precursor
materials together, heat the mixture to an appropriate temperature
and holding for a specified time to allow the layered oxide to
form. At this stage the layered oxide will be in a metastable
state. Simply changing the atmosphere to an inert atmosphere (for
example by putting the reaction chamber under a nitrogen
atmosphere) at the end of the formation of the material and cooling
to room temperature yield the oxygen deficient form of the material
without the need for secondary processing.
Generic Procedure to Make a Sodium Metal Electrochemical Test
Cell:
Electrochemical cells were prepared using conventional
electrochemical testing techniques. Materials were either tested as
powder, pressed pellets or as cast electrodes, each testing
methodology used is highlighted alongside the example materials. To
prepare an electrode of the test material the sample was prepared
using a solvent-casting technique, from a slurry containing the
active material, conductive carbon, binder and solvent. The
conductive carbon used is Super P C65 (Timcal). PVdF (e.g. Kynar)
is used as the binder, and NMP (N-Methyl-2-pyrrolidone, Anhydrous,
Sigma, UK) is used as the solvent. The slurry is then cast onto an
aluminium current collector using the Doctor-blade technique. The
electrode is then dried under Vacuum at about 80-120.degree. C. The
electrode film contains the following components, expressed in
percent by weight: 75% active material, 18% Super P carbon, and 7%
Kynar binder. Optionally, this ratio can be varied to optimise the
electrode properties such as, adhesion, resistivity and porosity.
The electrolyte comprises a 0.5 or 1.0 M solution of NaClO.sub.4 in
propylene carbonate (PC), and can also be any suitable or known
electrolyte or mixture thereof. A glass fibre separator (e.g.
Whatman, GF/A) or a porous polypropylene separator (e.g. Celgard
2400) wetted by the electrolyte is interposed between the positive
and negative electrodes forming the electrochemical test cell.
Typically, cells were symmetrically charged and discharged
galvanostatically at a rate of 5-10 mA/g.
The materials described herein can also be tested as powders, where
the active material is mixed with a conductive additive, either by
hand mixing or in a ball mill. The conductive carbon used is Super
P C65 (Timcal). The electro active mixture contains the following
components, expressed in percent by weight: 80% active material,
20% Super P carbon, this ratio can be varied to optimise the
mixtures properties such as, resistivity and porosity. The
electrolyte comprises a 0.5 or 1.0 M solution of NaClO.sub.4 in
propylene carbonate (PC). A glass fibre separator (e.g. Whatman,
GF/A) or a porous polypropylene separator (e.g. Celgard 2400)
wetted by the electrolyte is interposed between the positive and
negative electrodes forming the electrochemical test cell.
Typically, cells were symmetrically charged and discharged
galvanostatically at a rate of 5-10 mA/g.
Alternatively, materials described herein may also be tested as
pressed pellets, where the active material is mixed with a
conductive additive and a polymer binder, either by hand mixing or
in a ball mill. The conductive carbon used is Super P C65 (Timcal).
The electro active mixture contains the following components,
expressed in percent by weight: 80% active material, 10% Super P
Carbon, and 10% binder (PVdF or similar), this ratio can be varied
to optimise the mixtures properties such as, resistivity, porosity
and wetting behaviour of the pellet. The electrolyte comprises a
0.5 or 1.0 M solution of NaClO.sub.4 in propylene carbonate (PC). A
glass fibre separator (e.g. Whatman, GF/A) or a porous
polypropylene separator (e.g. Celgard 2400) wetted by the
electrolyte is interposed between the positive and negative
electrodes forming the electrochemical test cell. Typically, cells
were symmetrically charged and discharged galvanostatically at a
rate of 5-10 mA/g.
Cell Testing:
Electrochemical cells of materials prepared according to the
procedures outlined in Table 1 were tested using Constant Current
Cycling Techniques.
The cell was cycled at a given current density (ca. 5-10 mA/g)
between pre-set voltage limits as deemed appropriate for the
material under test. A commercial battery cycler from Maccor Inc.
(Tulsa, Okla., USA) was used. Cells were charged symmetrically
between the upper and lower voltage limits at a constant current
density. On charge sodium ions are extracted from the cathode and
migrate to the anode. On discharge the reverse process occurs and
Sodium ions are re-inserted into the cathode material.
Structural Characterisation:
All of the product materials were analysed by X-ray diffraction
techniques using a Bruker D2 phaser powder diffractometer (fitted
with a Lynxeye.TM. detector) to confirm that the desired target
materials had been prepared, and also to establish the phase purity
of the products and to determine the types of impurities present.
From this information it is possible to determine the unit cell
lattice parameters.
The operating conditions used to obtain the powder diffraction
patterns illustrated, are as follows:
Range: 2.theta.=10.degree.-90.degree.
X-ray Wavelength=1.5418 .ANG. (Angstroms) (Cu K.alpha.)
Step size: 2.theta.=0.02
Speed: 1.5 seconds/step
Diffraction patterns were collected using sample holders which
could allow measurement of diffraction under an inert atmosphere.
The sample holder contributes to the observed diffraction patterns
with large peaks centered at ca. 32.degree.=2.theta. and ca.
50.degree.=2.theta. and other smooth peak features can also be
observed.
Quantification of Oxygen Loss and Uptake:
The loss or uptake of oxygen could be readily quantified using
TGA-STA (Thermogravimetric analysis-simultaneous thermal analysis)
using a Perkin Elmer STA 6000 equipped with a passivated
Al.sub.2O.sub.3 crucible. To quantify the oxygen loss or uptake in
a layered oxide sample the sample was reheated to a temperature
less than or equal to the formation temperature. In a typical
experiment the sample was heated at a rate of 20.degree. C.
min.sup.-1 to a temperature less than or equal to the formation
temperature, the sample was held at temperature for a period in the
range 60 s-1 h to allow equilibrium of any metastable state, the
sample was cooled at a rate of 20.degree. C. min.sup.-1 to room
temperature. The heating and cooling protocol varied by sample as
did the combination of flowing gasses and the point at which gas
flows were changed. For example, to demonstrate oxygen loss in a
stoichiometric sample the sample was heated and cooled under a
constant nitrogen flow throughout the entire cycle. Similarly, to
demonstrate oxygen uptake in a oxygen deficient sample (Examples
8-12) the sample cycle was completed under a constant flow of air.
Control experiments were performed to confirm complete oxygen
re-uptake or retention of oxygen deficiency by heating under an
inert gas followed by cooling under oxygen or heating under air
followed by heating under an inert gas, respectively.
DETAILED DESCRIPTION OF THE INVENTION
Results
The present Applicant has found that not only are the oxidation
states of the metal constituents in the compounds of the present
invention a critical feature to the production of highly
electrochemically active compounds but they have also confirmed
that having particular transition metal constituents allows
variable oxidation states (i.e. oxidation states which are not
integers) in the same crystalline structure of the compound. It is
known that that there are several possible layered structural forms
which alkali metal/metal/oxides may adopt, including O3, P3 and P2.
The Applicant has shown that the oxidation states for the metal
constituents can allow oxidation states which are not integers to
be stabilised in many structural forms including O2, P3 and P2 via
the loss of oxygen from the material. This is achieved through
incorporating a reducible transition metal within the material
composition. The magnitude of the loss of oxygen may also be
controlled by tailoring the synthesis of these materials. The
applicant has also noted several benefits in the application of
these materials in electrochemical devices. Materials which show
oxygen deficiency generally show lower irreversibility on alkali
metal intercalation and de intercalation, they also show similar
capacity retention and similar intercalation potentials. The
slightly lower oxygen content also results in dilation of the unit
cell and realises a smaller volume change on electrochemical
cycling, leading to improvements in cell capacity retention.
The Invention Will Now be Described with Reference to the Example
Materials.
The invention relates to materials which have lost oxygen from
their ideal stoichiometry. In the simplest embodiment of the
invention oxygen loss may be induced in a stoichiometric layered
oxide by post processing. This aspect of the present invention will
now be described in reference to examples 1-11.
With reference to Example 1. The data shown in FIG. 1A shows the
Powder X-ray diffraction pattern of stoichiometric
NaNi.sub.0.25Na.sub.0.17Mn.sub.4/12Ti.sub.3/12O.sub.2 showing the
formation of an O3 layered oxide phase produced as described in
Example 1 for the formation of a stoichiometric layered oxide. Post
processing of this material leads to the loss of oxygen when post
processing via heating under an inert atmosphere is undertaken.
Oxygen loss in
NaNi.sub.0.25Na.sub.0.17Mn.sub.4/12Ti.sub.3/12O.sub.2 Oxygen loss
from the material was demonstrated using TGA-STA by reheating the
sample to a temperature of 800.degree. under a constant flow of
N.sub.2. The mass loss associated with reheating the material under
air and under N.sub.2 are compared in FIG. 1B. It can be seen that,
while mass loss occurs owing to oxygen loss as the sample is heated
in air, complete uptake of oxygen occurs in the sample heated in
air upon subsequent cooling leading to no overall mass loss. In
contrast the mass loss observed in the sample heated and
subsequently cooled under nitrogen is 0.57% this equates to a
stoichiometry if the mass loss is associated with oxygen of
NaNi.sub.0.25Na.sub.0.17Mn.sub.4/12Ti.sub.3/12O.sub.1.96
Electrochemically these materials show a benefit over samples which
do not show nonstoichiometry in oxygen. The first three
charge-discharge voltage profiles (Na-ion half cell Voltage against
a sodium metal anode [V vs Na/Na+] verses Cathode specific capacity
[mAh/g]) are shown in FIG. 1 D in which the sample produced by post
processing in air and under N.sub.2 are compared. It can be seen
FIG. 1 D that inducing oxygen deficiency within this material leads
to an slight decrease in the cycling capacity. However, there is
also a slight reduction in the irreversible capacity loss on the
first cycle with similar capacity fade over the first few cycles.
The average voltage of each cell is similar at 2.97 and 3.10 V vs
Na/Na+ for the sample post processed in air and nitrogen,
respectively. Calculation of specific energy density of the
materials yields values of 353 and 421 Wh/kg for sample post
processed in air and nitrogen, respectively. Showing that oxygen
non-stoichiometry in this material leads to a specific energy
density gain verses the stoichiometric oxide.
The reduction of first cycle loss is also demonstrated in FIG. 1 E
in which the differential capacity plot of the first
electrochemical cycle is shown.
With reference to Example 2 a material similar in structure to that
given in Example 1. The data shown in FIG. 2A shows the Powder
X-ray diffraction pattern of stoichiometric
NaNi.sub.0.25Na.sub.0.17Mn.sub.2/12Ti.sub.5/12O.sub.2 showing the
formation of an O3 layered oxide phase produced as described in
Example 2 for the formation of a stoichiometric layered oxide.
Oxygen loss from the material was demonstrated using TGA-STA by
reheating the sample to a temperature of 800.degree. under a
constant flow of N.sub.2. The mass loss associated with reheating
the material under air and under N.sub.2 are compared in FIG. 2B.
In this material a mass loss of 0.4% was realised upon heating and
subsequent cooling under N.sub.2, this equates to a stoichiometry
if the mass loss is associated with oxygen of N
Ni.sub.0.25Na.sub.0.17Mn.sub.0.166Ti.sub.0.416O.sub.1.97. It can be
seen that complete uptake of oxygen occurs upon cooling of the
sample heated in air leading to no overall mass loss. When compared
to the mass loss shown in Example 1 the magnitude of oxygen
deficiency within the material may be related to the content of a
reducible transition metal within the structure. To confirm that no
structural transitions occurred in the post processed sample a XRD
pattern of the material is shown in FIG. 2C in which it can be seen
that no structural transitions occur in the materials.
Electrochemically these materials can be differentiated in terms of
performance. The first two charge-discharge voltage profiles
(Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific
capacity [mAh/g]) are shown in FIG. 2 D in which the sample
produced by post processing in air and under N.sub.2 are compared.
It can be seen FIG. 2 D that inducing oxygen deficiency within this
material leads to an increase in the cycling capacity, and a
reduction in the irreversible capacity loss on the first cycle.
Also suggesting a relationship between transition metal elements
present and cycling capacity. The average voltage of each cell is
similar at 3.09 and 3.12 V vs Na/Na+ for the sample post processed
in air and nitrogen, respectively. Calculation of specific energy
density yields 285 and 369 Wh/kg for the sample post processed in
air or nitrogen, respectively. The increase in specific energy
density shown in the oxygen deficient materials is a distinct
advantage, in for example, an electrochemical cell. FIG. 2E
compares the cycling capacities and capacity retention of the
materials produced from Example 2. The Differential Capacity
Profiles for the 1.sup.st charge cycle (Differential Capacity
[mAh/g/V] verses Na-ion half Cell Voltage [V vs Na/Na+]) are shown
in FIG. 2 F in which the reduction of irreversibility in the
material can be attributed to a peak centered at 4.15 V vs Na/Na+,
this is usually attributed to oxygen loss in O3 layered oxide
materials. From the differential capacity plot shown in FIG. 2 F we
believe that oxygen deficient materials show lower oxygen loss on
cycling to high voltages which may also be beneficial in
manufacture of full cells based on these materials.
With reference to Example 3, this material is a compositional
variant of Examples 1 and 2. FIG. 3A shows the Powder X-ray
diffraction pattern of stoichiometric
NaNi.sub.0.25Na.sub.0.17Mn.sub.5/12Ti.sub.2/12O.sub.2 showing the
formation of an O3 layered oxide phase produced as described in
Example 3 for the formation of a stoichiometric layered. In this
material a mass loss of 0.56% was realised when post processed by
heating and cooling under N.sub.2 under the same conditions used in
Examples 1 and 2, this equates to a stoichiometry if the mass loss
is associated with oxygen of
NaNi.sub.0.25Na.sub.0.17Mn.sub.5/12Ti.sub.2/12O.sub.1.96. Which is
of similar magnitude to that observed in Examples 1 and 2.
Electrochemically Example 3 shows similar material properties to
those observed in Examples 1 and 2. However, this material may also
be differentiated in terms of performance to the stoichiometric
variant. The first four charge-discharge voltage profiles (Na-ion
half cell Voltage [V vs Na/Na+] verses Cathode specific capacity
[mAh/g]) are shown in FIG. 3 D in which the sample produced by post
processing in air and under N.sub.2 are compared. It can be seen
FIG. 3 D that inducing oxygen deficiency within this material leads
to an increase in the cycling capacity, and a reduction in the
irreversible capacity loss on the first cycle. The average voltage
of each cell is similar at 3.05 and 3.07 V vs Na/Na+ for the sample
post processed in air and nitrogen, respectively. Calculation of
specific energy density yields 367 and 451 Wh/kg for the sample
post processed in air or nitrogen, respectively. The increase in
specific energy density shown in the oxygen deficient materials is
a distinct advantage, in for example, an electrochemical cell. FIG.
3E compares the cycling capacities and capacity retention of the
materials produced from Example 2. The Differential Capacity
Profiles for the 1.sup.st charge cycle (Differential Capacity
[mAh/g/V] verses Na-ion half Cell Voltage [V vs Na/Na+]) are shown
in FIG. 2 F in which the reduction of irreversibility in the
material can be attributed to a peak centered at 4.15 V vs Na/Na+,
this is usually attributed to oxygen loss in O3 layered oxide
materials. From the differential capacity plot shown in FIG. 3F we
believe that oxygen deficient materials show lower oxygen loss on
cycling to high voltages which may also be beneficial in
manufacture of full cells based on these materials. We have
demonstrated that this oxygen loss consistently results in a
reduced first cycle loss and a slight increase in the average
potential of a cell constructed from oxygen deficient layered
oxides.
With reference to Example 4, this material is an example of a
layered oxide material which can be stabilized in an oxygen
non-stoichiometric form which contains less than one Na atom per
formula unit. FIG. 4A shows the Powder X-ray diffraction pattern of
stoichiometric Na.sub.2/3Ni.sub.1/3Mn.sub.2/3O.sub.2 showing the
formation of an P2 layered oxide product. In this material a mass
loss of 1.55% was realised when post processed by heating to
900.degree. C. and cooling under N.sub.2 this equates to a
stoichiometry if the mass loss is associated with oxygen of
Na.sub.2/3Ni.sub.1/3Mn.sub.2/3O.sub.1.91 which is of greater
magnitude to that observed in previous examples.
Electrochemically Example 4 shows similar material properties to
those observed in the O3 layered oxide materials described in
examples 1-3. However, this material can also be clearly
differentiated in terms of electrochemical performance to the
stoichiometric variant. The first four charge-discharge voltage
profiles (Na-ion half cell Voltage [V vs Na/Na+] verses Cathode
specific capacity [mAh/g]) are shown in FIG. 4 D in which the
sample produced by post processing in air and under N.sub.2 are
compared. It can be seen FIG. 4 D that inducing oxygen deficiency
within this material leads to an increase in the cycling capacity
97 mAh/g and 87 mAh/g for the non-stoichiometric and stoichiometric
samples. The average voltage is also increased by inducing oxygen
non stoichiometry in the sample and this raises from 2.50V vs
Na/Na+ to 2.71V vs Na/Na+ for the sample post processed in air,
respectively. Calculation of specific energy density yields 262 and
215 Wh/kg for the sample post processed in nitrogen and air,
respectively. FIG. 4E compares the cycling capacities and capacity
retention of the materials produced from Example 4 in which it can
be seen that the oxygen non-stoichiometric form of Example 4 shows
higher capacity retention over the first few electrochemical
cycles. The Differential Capacity Profiles for the 1.sup.st charge
cycle (Differential Capacity [mAh/g/V] verses Na-ion half Cell
Voltage [V vs Na/Na+]) are shown in FIG. 4 F in which it can be
seen that the irreversibility in this P2 material is similar
between the stoichiometric and non-stoichiometric with a similar
voltage profile.
With reference to Example 5, this material is a an example of a
layered oxide material which can be stabilized in an oxygen
non-stoichiometric form which contains less than one Na atom per
formula unit and forms a P2 layered structure. FIG. 5A shows the
Powder X-ray diffraction pattern of stoichiometric
Na.sub.0.76Mn.sub.0.65Co.sub.0.18Ni.sub.0.17O.sub.2 showing the
formation of an P2 layered oxide product as described in Table 1.
In this material a mass loss of 3.67% was realised when post
processed by heating to 900.degree. C. and cooling under N.sub.2
this equates to a stoichiometry if the mass loss is associated with
oxygen of Na.sub.0.76Mn.sub.0.65Co.sub.0.18Ni.sub.0.17 O.sub.1.76
which is of greater magnitude to that observed in previous examples
without a structure transition as shown in FIG. 5B.
Electrochemically Example 5 shows similar material properties to
those observed in the O3 layered oxide materials described in
examples 1-4. Again, this material can be clearly differentiated
electrochemically from the stoichiometric variant. The first cycle
charge-discharge voltage profiles (Na-ion half cell Voltage [V vs
Na/Na+] verses Cathode specific capacity [mAh/g]) are shown in FIG.
5 D in which the sample produced by post processing in air and
under N.sub.2 are compared. It can be seen FIG. 5 D that inducing
oxygen deficiency within this material leads to an increase in the
cycling capacity 120 mAh/g and 115 mAh/g for the non-stoichiometric
and stoichiometric samples. However, the average voltage is reduced
by inducing oxygen non stoichiometry in the sample and this reduces
from 2.8V vs Na/Na+ to 2.3V vs Na/Na+. Calculation of specific
energy density yields 268 and 321 Wh/kg for the sample post
processed in nitrogen and air, respectively. This reduction in
average voltage may be as a result of the significant proportion of
reduced elements present in the material when in its oxygen
non-stoichiometric form. The Differential Capacity Profiles for the
1.sup.st charge cycle (Differential Capacity [mAh/g/V] verses
Na-ion half Cell Voltage [V vs Na/Na+]) are shown in FIG. 5E in
which it can be seen that there is a contribution to capacity
originating from a low voltage reaction in the non-stoichiometric
material.
With reference to Example 6, this material is a an example of a
layered oxide material which can be stabilized in an oxygen
non-stoichiometric form which does not contain redox active +2
oxidation state metals and forms a O3 layered structure. FIG. 6A
shows the Powder X-ray diffraction pattern of stoichiometric Na
Fe.sub.0.5Ti.sub.0.125Mn.sub.0.125Mg.sub.0.25O.sub.2 showing the
formation of an O3 layered oxide product as described in Table 1.
In this material a mass loss of 0.18% was realised when post
processed by heating to 900.degree. C. and cooling under N.sub.2
this equates to a stoichiometry if the mass loss is associated with
oxygen of Na Fe.sub.0.5Ti.sub.0.125
Mn.sub.0.125Mg.sub.0.25O.sub.1.98, which is of similar magnitude to
that observed in previous
Again, Example 6 can be clearly differentiated electrochemically
from the stoichiometric variant. The first three cycle
charge-discharge voltage profiles (Na-ion half cell Voltage [V vs
Na/Na+] verses Cathode specific capacity [mAh/g]) are shown in FIG.
6 D in which the sample produced by post processing in air and
under N.sub.2 are compared. It can be seen FIG. 6 D that inducing
oxygen deficiency within this material leads to an increase in the
cycling capacity 95 mAh/g and 90 mAh/g for the non-stoichiometric
and stoichiometric samples. However, the average voltage is
increased by inducing oxygen non stoichiometry in the sample and
this increases from 3.12 to 3.16 V vs Na/Na+. Calculation of
specific energy density yields 300 and 280 Wh/kg for the sample
post processed in nitrogen and air, respectively. FIG. 6E compares
the cycling capacities and capacity retention of the materials
produced from Example 6 in which it can be seen that the oxygen
non-stoichiometric form shows higher capacity retention over the
first few electrochemical cycles. The Differential Capacity
Profiles for the 1.sup.st charge cycle (Differential Capacity
[mAh/g/V] verses Na-ion half Cell Voltage [V vs Na/Na+]) are shown
in FIG. 6F in which it can be seen that both the stoichiometric and
non-stoichiometric forms of this oxide show similar irreversibility
on the first electrochemical cycle.
Further to the embodiments of the invention described by Examples
1-6. Oxygen non-stoichiometry can be induced in materials by
altering the atmosphere under which the reaction product quench to
room temperature. Examples of this embodiment of the invention are
described in Table 1 by the materials of Examples 7-11. These
materials are produced by the "single step process" outlined above,
in which the precursor materials are mixed and heated in air for
the time period specified in Table 1. The atmosphere is then
changed to an inert atmosphere as specified in Table 1 and the
material is allowed to cool under the inert atmosphere, resulting
in an oxygen deficient material.
INDUSTRIAL APPLICABILITY
Electrodes according to the present invention are suitable for use
in many different applications, energy storage devices,
rechargeable batteries, electrochemical devices and electrochromic
devices. Advantageously the electrodes according to the invention
are used in conjunction with a counter electrode and one or more
electrolyte materials. The electrolyte materials may be any
conventional or known materials and may comprise of either aqueous
electrolyte or non-aqueous electrolytes or a mixture thereof.
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